Television cameras include lenses which focus images onto television imagers. The imagers respond to the light falling thereupon to produce time-sequential signals derived by recurrent raster scanning of the light-sensitive surface of the imagers. These time-sequential signals are intended to accurately represent the light produced by the image in the form of amplitude variations, with black levels typically being represented by zero signal, white levels by a positive voltage (although other connections may be used). The electrical signals generated by an imager such as a vidicon may not be linearly related to the amount of light falling thereupon, and television image reproducing devices such as kinescopes (picture tubes) do not linearly transduce the amount of video signals applied thereto into light, so nonlinear compensation is applied to the signals produced by a video signal source such as a camera, and this compensation is known as gamma (.gamma.) correction.
There are other conditions which result in production of video signals which do not accurately represent the illumination of the image. The condition known as flare results from internal dispersion of light within the lens, optical system and in the nominally black support structures for the lens, optical system and imager. This dispersion results from light which either enters the lens system at angles which do not result in the formation of an image focussed onto the imager or which enter at the proper angle and are dispersed by internal surfaces so as to impinge upon the nominally black (light-absorptive) support surfaces. However, these black surfaces do not completely absorb light, but re-reflect certain portions, with the result that a certain part of the ambient light suffuses the entire light-sensitive portion of the imager. Since this suffusing light is not focussed, it does not form a defined image but merely adds a certain amount of light to all points of the image. This shows up in the resulting signal as an apparent shift of the black-level towards white. The amount of light suffusing the surface of the imager depends upon the ambient light levels, and consequently the apparent black level changes in response to ambient light level.
In cameras using vidicon tubes and analog circuits, correction for the effect is accomplished by operating on the video signal produced by the camera. The operation consists of subtracting from the signal amplitude a predetermined amount which depends upon the average picture level (APL), which is assumed to be representative of the overall illumination of the scene and therefore representative of the signal offset attributable to flare. In an analog circuit, APL is simply established by an integrator coupled to the signal path. The integrator may be of the simple resistance-capacitance (RC) type or may be more sophisticated. Analog integrators have a response which is notoriously well known, one measure of which is the time constant (TC). An analog integrator may be made to have a TC which is long compared with a field or frame interval, whereupon it averages the signal values of the picture elements (pixels) of the current frame with a weighted average of the values of pixels of preceding frames (this latter weighting results from the discharge TC of the integrator which decreases the relative importance of signals applied in the relatively distant past).
Digital video signal processing is becoming more important because of the ability of digital circuits to perform signal processing functions more exactly than can their analog counterparts. Thus, functions requiring great exactitude are performed digitally. Notably, time-base correction in television video playback arrangements is almost universally performed digitally in high-quality equipment because the required signal delay function cannot be performed accurately by analog delays. Generally, it is desirable for accurate signal processing to perform a digital-to-analog conversion at the earliest possible point (as, for example, immediately following the camera tube or camera preamplifier) and to reconvert to analog just before the display function. Ideally, the signal would be both generated and displayed in digital (quantized) form, with the number of quantizing levels being selected to give the appearance of an analog display. It is therefore desirable to perform the flare correction and the associated APL determination in the digital domain.
In order to perform an APL measurement digitally in the same fashion in which the analog APL function is accomplished, it appears to be necessary to provide a field store having memory locations corresponding to the number of pixels per field or frame, to read the field store simultaneously with the present or current incoming pixel, add together the value of the current pixel and the stored value, and return the updated pixel to the field store to provide the weighted data, then add together the values of all the pixels and divide by the number of pixels averaged. This is an extremely large and complex circuit. The cost may be decreased by ignoring previous fields and merely adding together the values of all of the pixels of a field and dividing by the number of pixels, but even this circuit must add together the values of about one quarter-million pixels per field (assuming sampling of an NTSC signal at a rate of four times the color subcarrier frequency or 4.times.SC) and dividing by a like number.
It would be desirable to have a simple and convenient method of producing a digital signal representative of the APL for controlling a flare corrector and for other signal processing purposes.